Add gRPC API implementation

Changes:
- Rename the existing daemon to applaunchd-dbus, and add a new
  version that exposes a gRPC based implementation of the API based
  on the protobuf definition added in a top-level protos directory.
  Having both the D-Bus and gRPC APIs exposed by a single daemon was
  posing difficulties around startup dependencies stemming from D-Bus
  activation of the daemon.  Since the end goal is dropping the D-Bus
  API entirely, it is easier to just add a second daemon for gRPC that
  will eventually be the only one present.
- To facilitate building the two implementations, a significant amount
  of code refactoring has been done to move things from the D-Bus API
  implementing app_launcher.[ch] to systemd_manager.[ch] so the code
  can be reused.
- All use of the systemd D-Bus library except for the path encoding
  helper function has been replaced with GDBus.  The systemd interface
  wrapper code was generated with gdbus-codegen from XML files captured
  via introspection on a running system.  The motivation for this
  change was to avoid multithreading issues with sd_bus exposed when
  calling into it from the gRPC threads.
- The copyright headers in the source files have been tweaked to
  remove the Apache license boilerplate in favour of a SPDX license
  tag.

Notes:
- The gRPC API differs slightly from the D-Bus one in that it has a
  single status streaming RPC method as opposed to the separate signals
  for application started or terminated that the D-Bus API has.
- The gRPC API is currently unauthenticated, the aim is to circle back
  and implement authentication once a consensus can be reached on what
  mechanism should be used (fixed JWT configuration, OAuth, etc.).

Bug-AGL: SPEC-4559

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I828f38a58b60e9959162b98054982124d4fa4380

1 files changed, 17 insertions, 7 deletions

diff --git a/README.md b/README.md
index 83ea36d..4d0cabb 100644
--- a/README.md
+++ b/README.md
@@ -1,23 +1,33 @@
 AGL Application Launcher service reference implementation
 
 `applaunchd` is a simple service for launching applications from other
-applications. It exposes an interface named 'org.automotivelinux.AppLaunch' on
-on the D-Bus session bus and can be autostarted by using this interface name.
+applications.  It exposes a gRPC RPC interface as described in the file
+`protos/applauncher.proto`.  Additionally, there is a now deprecated
+`applaunchd-dbus`, which exposes a comparable version of the interface
+named 'org.automotivelinux.AppLaunch' on the D-Bus session bus and can
+be autostarted by using this interface name.
 
-This interface can be used to:
+The interface can be used to:
 - retrieve a list of available applications
 - request that a specific application be started by using the 'start' method
-- subcribe to the 'started' and/or 'terminated' signals in order to be
-  notified when an application started successfully or terminated
+- subcribe to a status signal (separate 'started' and/or 'terminated' signals
+  for the D-Bus implementation) in order to be notified when an application
+  has started successfully or terminated.
 
-For more details about the D-Bus interface, please refer to the file
-`data/org.automotivelinux.AppLaunch.xml`.
+For more details about the deprecated D-Bus interface, please refer to the
+file `data/org.automotivelinux.AppLaunch.xml`.
 
 Applications are enumerated from systemd's list of available units based on
 the pattern agl-app*@*.service, and are started and controled using their
 systemd unit.  Please note `applaunchd` allows only one instance of a given
 application.
 
+Note that while the gRPC and D-Bus implementations are comparable in
+functionality, they are not interoperable with respect to status notifications
+for applications started by the other interface.  It is advised that their
+usage not be mixed in the same image to avoid confusion around application
+window activation.
+
 AGL repo for source code:
 https://gerrit.automotivelinux.org/gerrit/#/admin/projects/src/applaunchd